Synthetic resin strings are extensively used for automatic packaging or binding machines, agricultural binders, hay balers and the like as well as for handicraft knitted articles.
The strings of this type are divided into twisted strings and untwisted strings.
Twisted strings are produced by mechanically twisting a stretched synthetic resin film or a split yarn or yarns obtained by longitudinally slitting a stretched synthetic resin film. However, the twisting operation is very inefficient and costly, entailing a low overall productivity and high manufacturing cost. The product itself, being twisted, has continuous protuberances, which tend to impede smooth movement of the string when it is paid out from a binder, producing a failure in binding operation. Moreover, the twist given to the string imparts stiffness to the string and impairs its shock-absorbing properties, with the result that the knot of binding string is liable to loosen.
Since the strings of the latter type are not twisted, they can be manufactured more efficiently and inexpensively than twisted strings, have improved smoothness and shock-absorbing properties and are lightweight and flexible, but if a stretched thermoplastic resin film is made into a thin string merely by being passed through a bundling ring or the like without twisting, the string will loosen and unfold to the original planar film, failing to retain shock-absorbing properties and bulkiness. Moreover, the string will then be prone to tearing. These objections make the string no longer serviceable for a binder. To eliminate such deficiencies, it has been practiced to cover the string with a film tube or to heat-set the string so as to prevent loosening and unfolding of the string. However when covered with the tube, the resulting double construction deteriorates the advantages of untwisted string described and entails reduced productivity and increased manufacturing cost, whilst the heat-setting treatment, unless conducted ingeniously, produces variations in the diametrical size of untwisted string. When subjected to excess heat-setting treatment, the string which is made of thermoplastic synthetic resin is hardened to exhibit poor flexibility, and when hardened up to its interior, the string fails to retain satisfactory shock-absorbing properties.